This guide will help you to understand where to camp better and how don’t die during waves.
Enemies spawn at randomly selected spawn points that are a sufficient distance from the player and out of line of sight. The final spawn position of a unit in a group might result in them spawning within sight but that’s the gist of the spawning setup. Enemies come in 3 waves, then the boss spawns. The end of each wave is marked by a slomo when you kill the last enemy.
For Extreme, I usually camp one of the back two corners behind the chapel. You get long lines of sight with indestructible walls you can poke in and out of for cover. Grab explosive barrels and place them in the rear chapel doors to get enemies clogged up there.
At the beginning of the fight while machinegunners are spraying the building, run a loop around and grab any supplies you need. There’s armor, a field kit, lots of TNT and frag shells, and more litterer about.
Get used to shooting TNT quick tosses to blow up tightly grouped enemies and shield cultists – much faster than swapping weapons around.
Be alert on the third wave when Ghosts and Harvesters can spawn.